1. Field of the Invention
This invention relates to a serial scanning type recording apparatus which produces a high quality image on a recording medium, such as a paper sheet, by means of a moveable recording carriage which faces and scans across the surface of a recording medium. More specifically, this invention relates to a shuttle type recording apparatus in which plural recording heads are mounted in spaced apart relation on a moveable recording carriage and which permits recording at high speed and with high accuracy. This invention is especially advantageous for the recording of images by the transfer of a color agent onto a recording medium based on image data; and it is suitable still more in connection with ink jet recording wherein a liquid recording ink is discharged as a color agent onto a recording medium. This invention, moreover, may be used for recording on many different recording media, for example, paper, cloth, leather, nontextile materials, overhead projector films and metal. One specific application for this invention is in a business machine such as a printer, a copy machine or a facsimile machine, as well as a manufacturing machine such as a textile printing machine.
2. Description of the Related Art
Serial scanning type recording devices which record by causing a recording head to scan across the surface of a recording medium while transferring an image to the recording medium have been widely used. Such devices are less costly than devices which record by means of a stationary recording head which extends over the whole width of a recording area.
A number of different serial scanning type recording devices, in which a color agent is transferred to a recording material, have been put into practice or proposed. These include devices in which a color agent is made to appear by the application of heat from a thermal head onto special thermal sensitive paper or by application of light onto a special light sensitive paper.
Moreover, devices which record by transferring a color agent from a recording head onto recording medium by a recording head, have been put into practice or have been proposed. These include impact recording devices which transfer ink onto a recording medium by impacting, with printing wires, an ink ribbon which has been soaked with liquid ink as a color agent. These color agent transfer devices also include thermal melt devices which record by application of heat from a thermal head onto an ink ribbon which contains a solid color agent. Heat sublimation transfer devices and ink jet devices which record by discharging liquid record ink onto a recording medium are also known.
Recently, color agent transfer recording devices have become widely used for recording onto plain paper. In particular, ink jet type recording devices are finding increasing use; and are expected to be used even more widely in the future in connection with printers, copy machines and other business machines because of their low noise, low running cost, ease of miniaturization, the possibility of plain paper recording, the ease of colorizing and so on.
Since the above described serial scanning type recording devices use a recording head which is mounted on a moveable carriage, a problem arises in that the recording speed is low.
Some of the techniques that have been proposed or put into practice in order to improve the recording speed of a serial scanning type image recording apparatus include: decreasing the number of scans by providing a wider recording head; shortening the scanning time by increasing the scanning frequency; and recording by bidirectional scanning. However, each of these techniques has disadvantages.
For example, a wider recording head is expensive to manufacture; and a wide recording head requires an expensive, high storage capacity print buffer. Also, where the device uses heat to effect a color agent transfer, some means must be provided to prevent deterioration of record quality and damage to the recording head from the high temperature used in the process. Further, in ink jet recording, where the recording head does not touch the recording medium, some means must be provided to prevent deterioration of recording quality which is caused by rolling or cockling of the recording medium as it becomes permeated with liquid ink.
When the scanning speed of the carriage is increased in order to increase recording speed, the carriage drive load is increased and a larger drive source is required.
Devices which use heat to transfer a color agent transfer onto a recording medium require some means to prevent deterioration of recording quality and damage to the recording head due to the high temperature of the recording head itself. In addition, ink jet recording devices which use liquid ink experience ink vibration within the recording head during ink discharge; and this causes deterioration of the recording quality.
Japanese Patent Laid-Open Publication No. 50-81437,U.S. Pat. No. 4,272,771, discloses methods and apparatus for increasing the recording speed of a serial scanning type image recording apparatus. As disclosed in that publication, a dot matrix high speed impact printer is provided with a plurality of printing heads which operate simultaneously to print each line of print. Also, in order to eliminate the delay caused by carriage return, the printing heads operate irrespectively of the direction in which the printing heads move. The dot matrix high speed impact printer disclosed in the above mentioned publication produces a "half step dotted" pattern row, which provides a significantly improved print quality. Also, the printer described in that publication uses a simple and highly reliable mechanical drive apparatus to move the printing head assembly accurately and rapidly.
The method and apparatus disclosed in the above mentioned publication permits simultaneous printing on both the left half and the right half of a printing line. In this case separate left side and right side printing head assemblies are supported on the same carriage mechanism. This arrangement allows printing to be carried out at almost double speed. The printing speed can be increased even more by providing more than two printing head assemblies and by operating the printing heads in both directions of movement. The above mentioned publication also discloses an arrangement for precise control of the movement and positioning of the printing head assembly. According to this arrangement a thin board is installed on the printer body and a light detector is installed on the printer head assembly. The thin board has transparent narrow slits arranged at roughly equal intervals therealong. Each slit corresponds to a row of print; and the slits have a length equal to about one half of the length of a print line.
The printing head carriage in the above described device is driven via a closed loop timing belt which in turn is driven by a motor. This arrangement provides adequate printing accuracy where a pixel density matrix of about 9.times.7 is to be used. However, where high resolution and high speed are needed, a problem arises due to expansion and contraction of the timing belt due to load variations on the belt as the printing head carriage is driven at different speeds. This problem is especially severe where several recording heads are mounted on the same carriage. This is because the additional heads, and the additional ink that must be supplied to these heads, increases the overall weight of the carriage; and consequently increases the load on the belt as scanning speed varies. Also, where the recording apparatus uses a recording head with a plural recording elements which are operated according to a time division sequence, the resulting variation in scanning speed causes a drastic effect on image quality. This is especially severe in ink jet recording apparatus which use a liquid ink and a recording head which does not touch the recording medium.
A further problem which occurs in the use of ink jet recording devices with plural recording heads is that some of the ink which is projected from ink discharge orifices on the printing head rebounds from the recording medium in the form of a mist and accumulates back on the recording head in the vicinity of the discharge orifices. As a result, ink jet recording systems generally require a recovery means to remove this accumulation. One technique that has been adopted is to provide a wiper blade of an elastic material, such as gum, to wipe the surface of the discharge orifice and remove this ink accumulation. Also, to prevent ink within the discharge nozzle of a recording head from evaporating and drying, a cap is arranged to cover the recording head during non-recording intervals. In addition, a suction pump may be provided in connection with the cap to maintain the flow of ink during non-recording intervals so that it does not experience an increase in viscosity. The suction pump may be arranged so as to recover the extra ink.
In some ink jet recording devices, which operate on an on-demand basis, not all of the several discharge orifices are in use at the same time; and some of the orifices may not be use for a considerable length of time, particularly in the case of color recording devices where one color may used for only a small portion of the printing. Also, in a plural head color recording device, one recording head may not be used for a long period of time, depending on the recording instructions and data supplied to it. Now, when the carriage scans or stops in a position where the surface of the ink discharge orifice is not capped, the ink within and surrounding the orifice begins to dry and causes a deterioration in the quality of the subsequently recorded image. In order to prevent this phenomenon in an ink jet recording device, an arrangement is provided to cause a discharge of ink at some predetermined location and time which is unrelated to the actual recording. This kind of ink discharge action is called a preliminary discharge. By maintaining the discharge of ink from within the discharge orifice, fresh ink is always available for recording and high quality recording can be maintained. In order to keep the preliminary discharge ink from scattering and dirtying the recording material and the inside of a recording apparatus, the preliminary discharge is generally arranged to occur within the cap while it covers the head at a preliminary discharge position of the head. The provision of means for maintaining the cleanliness of plural recording heads in ink jet recording devices severely complicates the construction of the heads.